Implications of recent cosmic ray results for ultrahigh energy - - PowerPoint PPT Presentation

Subir Sarkar

Implications of recent cosmic ray results for ultrahigh energy neutrinos

Neutrino 2008, Christchurch 31 May 2008

‘knee’ – galactic source limit? ‘ankle’ – extragalactic source? Second ‘knee’ ?

Cosmic rays have energies upto ~1011 GeV … and so must cosmic neutrinos

(Courtesey: Ralph Engel)

I will focus on the Auger results alone since its hybrid detection ability enables reliable determination of both the energy and the acceptance

10th May 2007, E ~ 1010 GeV

The flux is suppressed beyond ~EGZK [arXiv:0706.2096] … but is it due to the GZK effect? P + γ2. 7 Κ → Δ

+ 1232 →

p + π

The arrival directions correlate with nearby AGN [arXiv:0711.2256]

… but are AGN really the sources? Recent cosmic ray results

At these high energies the sources must be nearby … within the ‘GZK horizon’ … and the observed UHECRs should point back to the sources

Dolag, Grasso, Springel & Tkachev (2003) Harari, Mollerach & Roulet (2006)

Are there any plausible cosmic accelerators for such enormous energies?

Whatever they are, the observed UHECRs should point back to them!

Easier to accelerate heavy nuclei

Active galactic nuclei

TeV γ-rays have been seen from AGN, however no direct evidence so far that protons are accelerated in such objects … renewed interest triggered by possible correlations with UHECRs - e.g. 2 Auger events within 30 of Cen A

Estimate

• f ν flux

from p-p:

Halzen & Murchadha [arXiv:0802.0887]

⇒ 0.02-0.8 events/km2 yr

The primaries are not photons

[arXiv:0712.1147]

… but may be heavy nuclei

[arXiv:0706.1495]

… as predicted by ‘top-down’ models … easier to accelerate to such energies Recent cosmic ray results

GZK interactions of extragalactic UHECRs on the CMB

(“guaranteed” cosmogenic neutrino flux … but may be altered significantly if the primaries are heavy nuclei rather than protons as is suggested by Auger data)

What are the expectations for the diffuse neutrino background?

UHECR candidate accelerators (AGN, GRBs, …)

(“Waxman-Bahcall flux” - normalised to extragalactic UHECR flux … sensitive to ‘cross-over energy’ above which they dominate, also to composition)

‘Top down’ sources (superheavy dark matter, topological defects)

(motivated by AGASA events - predicts that photons dominate over nucleons … all such models are now ruled out by new photon limit from Auger)

→ energy spectrum determined by QCD fragmentation → composition dominated by photons rather than nucleons → anisotropy due to our off-centre position

(Berezinsky, Kachelreiss & Vilenkin 1997; Birkel & S.S. 1998)

It was proposed that UHECRs are produced locally in the Galactic halo from the decays of metastable supermassive dark matter particles

Simulation of galaxy halo (Stoehr et al 2003)

These can be produced at the end of inflation by the changing gravitational field

X → partons → jets (→ ~90% ν, 8% γ + 2% p+n)

Perturbative evolution of parton cascade tracked using (SUSY) DGLAP equation … fragmentation modelled semi-empirically The fragmentation spectrum shape matches the AGASA data at trans- GZK energies … but bad fit to Auger Modelling SHDM (or TD) decay Most of the energy is released as neutrinos with some photons and a few nucleons …

(Toldra & S.S. 2002; Barbot & Drees 2003; Aloisio, Berezinsky & Kachelreiss 2004) ν γ p + n

Such models are falsifiable … and now ruled out by photon limit from Auger!

The “guaranteed” cosmogenic neutrino flux

But what if the primaries are heavy nuclei? … boosts νe flux but can suppress the νμ flux Hooper, Taylor, S.S. (2004); Ave et al (2004)

(Courtesey: David Waters)

UHE protons lose energy mainly

• n the cosmic microwave

background (CMB) … but UHE nuclei lose energy mainly on the cosmic infrared background (CIB)

(now well-constrained by γ-ray data)

Hooper, S.S. & Taylor [astro-ph/0608085]

Small uncertainty due to unknowns in evolution of CIB and of source density with cosmic redshift … note that all observed cosmic rays come from z < 1

• The degree of suppression depends critically on the maximum

energy to which cosmic rays are accelerated

Fe: Emax=1022.5 eV

56Fe + γCMB/CIB →

55Mn + p,

55Mn + γCMB/CIB →

54Mn + n, … In order to contribute to the cosmogenic neutrino flux, the photo-disassociated protons must exceed the GZK cutoff in energy, hence the original nuclei must have energies > EGZK x A

Emax=1021.5 eV

p He

Fe

O

Hence the (lower energy) νe flux is boosted but the (higher energy) νμ flux is suppressed ⇒

• verall reduction in event rate

(but very sensitive to Emax!)

Obtain solution in excellent agreement with Monte Carlo simulations …

Hooper, S.S. & Taylor (2008)

Analytic solution to photodisintegration of heavy cosmic ray nuclei on the CIB

⇒

Anchordoqui, Hooper, S.S. & Taylor [arXiv: 0709.0734]

Heavy nuclei as primaries are consistent with the observed energy spectrum and composition … but predict a smaller cosmogenic flux

Hence these estimated (cosmogenic ν) rates should now be considered as upper limits

Halzen and Hooper [astro-ph/0605103]

The sources of cosmic rays must also be neutrino sources

Making a reasonable assumption about επ allows this to be converted into a flux prediction (would be higher if extragalactic cosmic rays become dominant at energies below the ‘ankle’ )

(Courtesey: David Waters)

Anchordoqui, Hooper, SS & Taylor, astro-ph/0703001

We have studied whether high energy nuclei can survive photodisintegration by the (known or estimated) photon fields in suggested extragalactic sources of cosmic rays … the answer is no for GRBs, yes for starburst galaxies, and in between (energy-dependent) for AGNS

Hence the effect on the expected WB flux depends on what the actual sources are … e.g. a bi-modal model would yield: E2 φ ν ~ 10

− 9 cm-2 sec-1 st-1

Limits from AMANDA/IceCube so far constrain the WB flux only in models where extragalactic sources are assumed to dominate from as low as ~1018 eV (Ahlers et al 2005)

Upper limits to UHE cosmic neutrino fluxes

To see the cosmogenic ν flux will require larger detection volume (ANITA, …)

Auger can see ultra-high energy neutrinos as inclined deeply penetrating showers Rate ∝ cosmic neutrino flux, ∝ ν-N #-secn

An unexpected bonus – UHE neutrino detection with air shower arrays

Auger can also see Earth-skimming ντ → τ which generates upgoing hadronic shower

Rate ∝ cosmic neutrino flux, but not to ν-N #-secn

No neutrino events yet … but getting close to “guaranteed” cosmogenic flux (NB: ~To do this we must know ν-N cross-section at ultrahigh energies)

[arXiv:0712.1909]

Cooper-Sarkar & S.S. [arXiv:0710.5303]

Deep inelastic e-p scattering at HERA has probed the parton distribution functions down to very low xBjorken and very high Q2 … enables more reliable prediction of the UHE neutrino-nucleon cross-section (in the perturbative SM) using DGLAP evolution of the PDFs (at next-to- leading order, and including heavy quark corrections)

ν-N deep inelastic scattering

As the gluon density rises at low x, non-perturbative effects become important … a new phase of QCD - Colour Gluon Condensate - has been postulated to form

This would suppress the ν-N #-secn below its (unscreened) SM value

The steep rise of the gluon density at low-x must saturate (unitarity!) ⇒ suppression of the ν-N #-secn

Beyond HERA: probing low-x QCD with DIS of cosmic neutrinos

Extrapolation using HERA data

The ratio of quasi-horizontal (all flavour) and Earth-skimming (ντ) events measures the cross-section

Anchordoqui, Cooper-Sarkar, Hooper, S.S. [hep-ph/0605086]

Summary

Cosmic ray astronomy has been born … The sources of UHE cosmic rays must also emit neutrinos! The detection of UHE cosmic neutrinos is eagerly anticipated …but to do physics will likely require multi-km3 detectors Neutrino observatories will provide an unique laboratory for new physics, both in and beyond the Standard Model

“The existence of these high energy rays is a puzzle, the solution of which will be the discovery of new fundamental physics or astrophysics”

Jim Cronin (1998)